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Help with MYOG Steripen Freedom solar charger
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Connor C
(athousandtimes) - F

Locale: California
Help with MYOG Steripen Freedom solar charger on 06/02/2013 16:19:14 MDT Print View


My girlfriend and I would like to make a solar charger for her Steripen Freedom. The Freedom has an internal lithium polymer battery that can be recharged through a USB port.

Without doing very much homework we bought a 6V, 200mA, 1.2W solar cell, Schottky diode, and female USB port for $9 on ebay.

Here's a picture of the cell, diode, and USB port:

solar cell, diode, and USB port

I don't know if this helps, but this is what is says on the back of Steripen Freedom's wall charger.

Steripen Wall Charger

Has anyone else built a solar charger for a Steripen Freedom?

I don't really know anything about circuitry or electronics and was wondering if someone could point me in the right direction in terms of building the charger, where to attach the diode, if a regulator or additional diode is necessary, etc... I have access to a soldering gun.

Thank you!


Edited by athousandtimes on 06/02/2013 16:33:42 MDT.

Nathan Hays

Locale: San Francisco
Re: Steripen solar on 06/02/2013 16:44:52 MDT Print View

Lots of things to consider.

The wallwart is rated for 1000mA = 1 Amp.
I doubt the solar panel puts out one fifth of that.
The Steripen may require more current than the panel provides.
You just have to try.

The diode may be duplicated in the Steripen.
A diode unnecessarily reduces the panel efficiency if not needed.
If you connect a 100 resistor across Usb pins 4 and 1 and you see 3 or more volts, then the Steripen does not have one and you need to add yours.
Another way to test is to try and light a headlamp from pins 1&4.
Try both polarities.
If it doesn't light, the Steripen already has a protection diode.

The diode has an arrow into a bar: (+) -->|-- (-)
There may not be an arrow, just the bar.
One output of the panel is (+).
The diode (+) (arrow) end goes there.
The (-) (bar) end of the diode goes to Usb pin 1.
The (-) end of the solar cell goes to Usb pin 4
Usb pin 1 should be marked with '1' or a dot.

The Steripen may or may not require pins 2 and 3 to be held to 2.3v and 2.7v.
That requires four resistors to create those two voltages.
Try without first.

Connor C
(athousandtimes) - F

Locale: California
Regulator needed? on 06/02/2013 20:36:59 MDT Print View

Thank you for your comments, they're much appreciated.

Do you know if will I need a regulator of some kind to protect the Steripen from damage?


Nathan Hays

Locale: San Francisco
Re: Regulator on 06/02/2013 22:00:47 MDT Print View

I doubt it. An extra volt shouldn't harm any lithium charger. I looked at several chips that might be susceptible, but all were at least 6V max rated. And if there is a diode in line, it drops the voltage 0.7V (for most diodes) so you've got only 5.3V at the Steripen. The USB spec is 5.25V max, close enough that there is no issue.

Put the diode in line and check the voltage.

The real issue may be that the Steripen charger only works with higher current, but again I doubt it. That is a problem for iPhone charging because the phone keeps bleeping in and out of charge mode when the current isn't strong enough or the panel passes through momentary shade. Then you need a buffer battery and circuitry to manage it.